Structural analysis of F18 fimbriae expressed by porcine toxigenic Escherichia coli

The F18 fimbriae expressed by porcine toxigenic Escherichia coli strains ar e 1- to 2-mm-long filaments that mediate the adhesion of the bacteria to en terocytes. The backbone of these fimbriae is built from a major structural 15.1-kDa protein, FedA. The structure of isolated negatively stained F18 fi mbriae imaged by dark-field scanning transmission electron microscopy (STEM ) was resolved to approximately 2 nm. Analyzing their helical symmetry show ed the axially repeating units to alternate in a "zigzag" manner around the helical axis with an axial rise of 2.2 nm. Two repeating units give rise t o the observed 4.3-nm helical repeat, which is practically identical to the pitch of the one-start helix formed. Additionally, an axially repeating pa ttern with a 27-nm spacing was found on rotary-shadowed fimbriae. Mass-per- length determination of unstained F18 fimbriae by STEM revealed the axially repeating unit to have a molecular mass of 25.4 kDa, indicating that it is a FedA. monomer, with the difference in mass arising from the minor subuni ts, FedE and FedF, The presence of the latter two proteins might cause the observed 27-nm axial pattern. (C) 2000 Academic Press.